This invention relates to an improved coated carbide cutting tool insert, and more particularly to the combination of a support structure and a three layered coating on the insert. The inner and outer layers on the insert are of a nitride material. The intermediate layer is a hard wear resistant carbide material and the inner nitride layer cooperates with a cobalt enriched zone in a cobalt cemented carbide insert to more effectively support the other two layers.
1. Field of the Invention
Coated cemented carbide inserts have been effectively utilized in many metal removal applications for a number of years. Basically, they are composite materials prepared by chemical vapor depositing (CVD) processes which provide a thin layer or coating of a hard wear resistant material, for example, titanium carbide (TiC), on a hard metal substrate surface such as a tungsten cemented carbide (WC). In some instances, the TiC layer is preceded on the substrate by an underlayer of various materials, titanium nitride, TiN, for example, and an overlayer of TiN, aluminum oxide Al.sub.2 O.sub.3 and the like. Multilayer inserts have found application in a broad range of metal cutting applications, and various layers and their materials may be selected to suit the particular metal removal application.
2. Description of the Prior Art
The manufacture of coated cemented carbide tools and inserts includes a number of chemical and physical requirements. The coating layers utilized must be chemically stable and physically wear resistant in various metal cutting and wearing applications. The composition and thickness of these coatings are quite relevant because they must not easily spall or crack, and, they must be securely bonded to and supported by the insert substrate. Titanium carbide layers, TiC, titanium nitride layers, TiN, titanium carbonitride layers, TiCN, and aluminum oxide layers Al.sub.2 O.sub.3 in numerous combinations, structures, and ordered layers are known in the art. However, titanium carbide, TiC, has emerged as the predominant wear surface. Accordingly, titanium carbide layers have been laid down on various substrates by a number of different process to perform as a hard wear surface.
When there are two or more dissimilar layers, the kind of material in the first layer, and supporting relationship between the first layer and a cemented carbide substrate are most important from a structural point of view. Since TiC is the more important layer, its relationship to the cemented carbide substrate is critical. For this reason the TiC layer is usually found as a thinner layer next adjacent a cemented carbide substrate, particularly one containing TiC and some advantage is taken of the affinity of the two carbides. Because of the noted superiority of TiC layers as the predominant hard wear resistant layer, some attention has been given to ways and means to use thicker TiC layers and also additional individual layers of other materials which contribute to the effectiveness of the TiC layer. However, the result of the use of plural and thicker layers is a generally weakening of the structure.
With respect to adequately supporting the hard wear resistant outer layers on cemented carbide substrates, and effectively supporting more and thicker layers, recent improvements include a metallurgical gradation of the layers at their junctures which define transitional zones incorporating elements from each adjacent layer. In the case of a cobalt cemented carbide substrate this gradation relates to a surface zone or region of the substrate which is enriched in cobalt in that it contains a higher average concentration of cobalt than found elsewhere in the cemented carbide. It has been discovered that this enriched zone may be used not only to provide improved strength and bonding characteristics, but also to combine a first layer of a material other than TiC which contributes additionally to the structural integrity of subsequent layers.